Recent studies have shown that there are many common features of the plant and animal defense responses to pathogen attack, suggesting ancient origins. We have been using a genetic approach to study host defense responses in the model genetic organisms Arabidopsis thaliana and Caenorhabditis elegans. A set of Arabidopsis enhanced disease susceptibility (eds) mutants has been used to define a variety of defense response pathways that involve the low molecular mass signaling molecules salicylic acid (SA), jasmonic acid (JA) and ethylene (ET). eds16, which accumulates much lower levels of SA in response to pathogen attack than wild- type plants, has been the most informative mutant in our current studies. The cloning of EDS16 has shown that it encodes isochorismate synthase (ICS), an enzyme involved in salicylic acid biosynthesis in bacteria, but not previously known to be involved in SA biosynthesis in plants. Our work with C. elegans primarily involves the broad host-range bacterial pathogen Pseudomonas aeruginosa, which is infectious in plants, insects, and mice and kills C. elegans. We have recently shown that it is possible to readily isolate C. elegans mutants that exhibit enhanced susceptibility to pathogen (esp) mutants, analogous to the Arabidopsis eds mutants isolated previously. We believe that direct experimental comparison of innate immunity in Arabidopsis and C. elegans will be highly synergistic. There are four specific aims. First, we propose to verify that EDS16 encodes an enzyme with ICS activity and to test the hypothesis that Arabidopsis has at least two SA biosynthetic pathways, one involving isochorismate and a second that involves phenylalanine as intermediates. Second we propose to clone and further characterize the Arabidopsis EDS5, EDR5, DDE1, EDS14, EDS15, and NPR2 defense-related genes and to determine the roles of these genes in defense-response signaling pathways. Third, we propose to clone and characterize the C. elegans esp-2 and esp-8 genes, which we have tentatively identified as defense-related genes, and to isolate additional C. elegans esp mutants. Finally, we propose to use genomic and reverse genetic approaches, including DNA microarray analysis, to identify C. elegans and Arabidopsis defense-related genes.